A motor vehicle may include a belt integrated starter-generator (BISG) that can be used as a motor to start the engine of the motor vehicle, operate as a generator to generate electrical energy for use by the motor vehicle and in some cases assist with driving of the motor vehicle by for example providing torque assist to the engine.
The BISG may also be used to recuperate energy from the motor vehicle during a coast down of the motor vehicle.
A coast down is a situation where the operator of the motor vehicle is not requesting the delivery of torque from the engine.
In order for energy to be recuperated by the BISG the engine always has to turn for the BISG to recuperate kinetic energy and with the driveline between the engine and the road wheels remain intact and connected/engaged.
In order to increase the kinetic energy that can be recovered by the BISG and therefore increase fuel economy, engine friction may be decreased to a lowest possible value. One method to reduce engine friction is cylinder deactivation. Test work has indicated that engine friction can be almost halved if all of the cylinders of an engine are deactivated (e.g., by deactivating intake and exhaust valves of the engine cylinders) during a coast down. Therefore the deactivation of all cylinders of an engine may produce significant fuel economy improvements due to the increased energy that can be recuperated by the BISG into the electrical storage device (battery) of the motor vehicle by reducing the engine friction losses.
However, the inventors herein also recognize that in the case of a turbocharged engine the deactivation of all of the cylinders of the engine during a coast down event will result in several problems.
Firstly, the speed of the turbocharger will rapidly drop during the deactivation of some or all cylinders due to the reduction or lack of gas flow through the engine. If the deactivation extends for a long period of time, the turbocharger may eventually stop rotating and this may lead to a delay until torque is delivered to the driver when the coast down ends and the driver requests torque from the engine.
Secondly, if a turbocharger stops completely its durability is impaired due to the loss of hydrodynamic pressure in the bearings of the turbocharger.
Thirdly, if a turbocharger is stopped there will be a very high acceleration of the turbocharger when torque is eventually demanded and this may produce high stress, particularly in the blades of the turbine of the turbocharger and increase the rate of fatigue of the blades of the turbine wheel. This may lead to a premature fracture of one or more blades of the turbocharger with undesired consequences to the turbocharger and potentially the engine.
In some examples approach are described herein that may in part provide a method of controlling the operation of a turbocharged engine of a motor vehicle having a belt integrated starter generator used for recuperating energy from the motor vehicle that provides improved energy recuperation from the motor vehicle with a reduced risk of damage occurring to the turbocharger.
In one example, a method of controlling a turbocharged engine of a motor vehicle having a belt integrated starter-generator driveably connected to the engine to selectively recuperate energy from the motor vehicle during a coast down of the motor vehicle wherein the method comprises deactivating a predefined number of cylinders of the engine when a coast down event commences and energy recovery is required, operating the belt integrated starter-generator as a generator to recuperate energy from the motor vehicle, comparing an indication of current speed of the turbocharger with a predefined minimum turbocharger speed limit and reactivating at least one cylinder of the engine if the indicated speed of the turbocharger is less than the predefined speed limit.
The predefined number of cylinders may be all of the cylinders that are equipped with a cylinder deactivation mechanism. Advantageously, all of the cylinders of an engine are deactivatable if the energy recuperation is to be maximised.
The method may further comprise maintaining the cylinders of the engine deactivated if the indicated speed of the turbocharger is above the predefined turbocharger speed limit, the coast down event is continuing and energy recovery is still required.
The method may further comprise operating at least one cylinder of the engine in an active state when the coast down event ends.
The number of cylinders operated in the active state may be based upon the magnitude of the torque demand for the engine.
There may be two predefined turbocharger speed limits, a high turbocharger speed limit for use when cylinders of the engine are first deactivated and a lower turbocharger speed limit for use after at least one cylinder of the engine has been reactivated.
The method may further comprise increasing the number of cylinders reactivated in an iterative manner to maintain the turbocharger speed above the lower turbocharger limit.
According to a second aspect there is provided a motor vehicle having a turbocharged engine, a belt integrated starter-generator driveably connected to the engine to selectively recuperate energy from the motor vehicle during a coast down of the motor vehicle, an energy storage system to store energy recuperated by the belt integrated starter-generator, a cylinder deactivation system to selectively deactivate the cylinders of the engine, an electronic controller to control the operation of the belt integrated starter-generator, the energy storage system and the cylinder deactivation system in response to an input indicative of the speed of rotation of the turbocharger and an input indicative of the operating state of the motor vehicle wherein, when the input indicative of the operating state of the motor vehicle indicates that a coast down state is present, the controller is operable to use the belt integrated starter-generator as a generator to recuperate energy from the motor vehicle, use the cylinder deactivation system to deactivate a predefined number of cylinders of the engine when a coast down event commences and energy recovery is required and is further operable to use the cylinder deactivation system to reactivate at least one cylinder of the engine if the input indicative of the speed of rotation of the turbocharger indicates that the speed of the turbocharger is less than a predefined speed limit.
The predefined number of cylinders may be all of the cylinders that are equipped with a cylinder deactivation mechanism.
The electronic controller may be operable to maintain the cylinders of the engine that have been deactivated in the deactivated state provided the input indicative of the speed of rotation of the turbocharger indicates that the turbocharger speed has not dropped below the predefined turbocharger speed limit and the input indicative of the operating state of the motor vehicle indicates that a coast down state is still present and energy recovery is still required.
The electronic controller may be further operable to operate at least one cylinder of the engine in an active state when the input indicative of the operating state of the motor vehicle indicates that the coast down event has ended.
The number of cylinders operated by the electronic controller in the active state may be based upon the magnitude of a torque demand for the engine.
There may be two predefined turbocharger speed limits stored in the electronic controller, a high speed limit for use when the cylinders of the engine are first deactivated and a second lower speed limit for use after at least one cylinder of the engine has been reactivated.
The electronic controller may be operable to increase the number of cylinders reactivated in an iterative manner to maintain the turbocharger speed above the lower turbocharger limit.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description.
It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.